1. Field of the Invention
The present invention relates to a digital data transmitting/receiving system for transmitting motion image or picture data coded by the International Standard of ISO/IEC, MPEG-2 or the like for coding motion picture data with high efficiency. The present invention also relates to a digital data decoder device for decoding coded motion picture data and outputting it therefrom and a receiver for receiving displayed digital data. Incidentally, a description will be made below of an example in which the present invention is applied to a digital broadcast. However, the present invention is not necessarily limited to this. The present invention can be also applied to an example related to the transmission and reception of data such as data communications or the like in a similar mechanism.
2. Related Art
Due to an increase in the amount of digital motion image or picture data, high-efficiency coding means performs compression thereon to eliminate redundancy or the like and thereafter transmits or records the coded motion picture data, whereby the cost of transmission or recording can be reduced. As the high-efficiency coding means, there is well known-an MPEG-2 system standardized by ISO/IEC/JTC1/SC29/WG11.
As to the coding based, on the MPEG-2 system, respective frames of motion picture data are separated into an I frame (Intra Picture) coded without having a frame (reference frame) referred as a predicted value, a P frame (Predictive Picture) with only frames lying in the forward direction in display order as reference frames, and a B frame (Bidirectional Picture) with each frame lying in the forward direction and each frame lying in the backward direction as reference frames. Upon actual coding, two reference frames lying in the forward and backward directions are required to exist upon decoding the B frame, and the data coding is performed after the sequence of frames is skillfully interchanged.
While a decoder device successively decodes coded motion picture data transmitted in coding order, it is necessary to temporarily store the decoded data in a memory and re-arrange or sort the same in accordance with display order. It is also necessary to use decoded data of the I and P frames as reference data upon the subsequent decoding of B frame. The memory is required to certainly store motion picture data corresponding to two frames therein. The re-arrangement or sorting of the data in frame order is performed using each memory corresponding to the two frames. Since the data is coded in one frame unit, the frame data cannot be displayed simultaneously with decoding even in the case of the B frame when one frame is made up of two interlaced fields as in the case of a television signal. It is necessary to convert the frame data to field data sorted in the order of scanning lines. Even for this purpose, a memory corresponding to about one frame is required.
Further, image or picture signals compressed by the MPEG-2 system are classified into several categories. Special emphasis is placed on the two categories called a “main level (ML)” and a “high level (HL)” in particular from an application standpoint.
The main level corresponds to the NTSC system of horizontal 720 pixels×vertical 480 scanning lines×frame frequency of 30 Hz and is adopted for a satellite digital broadcast. The high level corresponds to a high-definition image or picture expressed in horizontal 1920 pixels×vertical 1080 scanning lines×frame frequency of 30 Hz. It has been determined to be adopted for a US ground wave digital broadcast intended to provide high picture-quality broadcasting services. The US ground wave digital broadcast has been introduced in the May 1997 issue, pp 47-53 of Nikkei Microdevice, for example.
The capacity of a memory required to decode the coded motion picture data based on the MPEG-2 system is equivalent to one added with that for a coded image or pictorial data buffer for temporarily storing a coded image upon decoding in addition to the capacity corresponding to the three frames. The capacity of the coded image data buffer (VBV) corresponds to 1,835,008 bits in main level and 9,781,248 bits in high level. Even in the case of combinations of coding devices and decoder devices different in manufacturer, this capacitive value is determined by the MPEG-2 system as the required minimum capacity necessary to always maintain the assurance of suitable coding/decoding.
As also introduced in the May 1997 issue, pp 47-53 of Nikkei Microdevice, the picture formats of the coded motion picture data range over many divergences. Thus, the display of decoded motion picture data corresponding to these all picture formats on a specific monitor needs to use a display device capable of displaying all these picture formats or utilize a format converting device between a decoder device and a display. The latter is advantageous to reduce the cost of the monitor.
Further, the satellite digital broadcast and the ground wave digital broadcast are not independent services for users who receive these broadcasts and enjoy the same. It is very natural that there is a demand that the users desire to enjoy even the conventional analog broadcast through the same television receiver. It is necessary to meet this demand.